1. Field of the Invention
In one embodiment, the invention relates to a close-tolerance expandable centralizer to dispose an expandable pipe string to a desired position within a borehole. One embodiment of the invention is directed to an apparatus and a method to deploy an expandable centralizer, after it is received on an expandable pipe string and run into a borehole, to generally dispose the expanded pipe string to a desired position within the borehole.
2. Background of the Related Art
Centralizers are commonly secured at spaced intervals along a pipe string to provide radial stand-off of the pipe string from the interior wall of a borehole in which the pipe string is subsequently installed. The term “pipe string,” as used herein, may refer to a casing string, a drill string, or any other tubular. A centralizer generally comprises a pair of generally aligned and spaced-apart collars defining a bore therethrough for receiving the pipe string, and a plurality of angularly-spaced ribs that project radially outwardly from the pipe string to provide the desired stand-off from the interior wall of the borehole.
A centralizer can center the pipe string within the borehole to provide a generally uniform annulus between the exterior surface of the pipe string and the wall of the borehole. The centering of the pipe string within the borehole can promote uniform and continuous distribution of cement slurry within the annulus during cementing of the pipe string within a targeted interval of the borehole. Uniform cement slurry distribution can result in a cement liner that better reinforces the pipe string, isolates the pipe string from corrosive formation fluids, and prevents unwanted fluid flow between penetrated geologic formations.
A bow spring centralizer employs flexible bow springs to provide variable stand-off from the borehole. Bow spring centralizers can include a pair of axially-spaced and generally aligned collars coupled one to the other by a plurality of bow springs. The flexible bow springs can be biased toward a deployed configuration to bow radially outwardly and away from the axis of the bore through the centralizer to engage the wall of the borehole and can center a pipe string received through the bores of the collars. Configured in this manner, the bow springs provide stand-off from the wall of the borehole, and flex or collapse radially inwardly to accommodate restrictions and/or irregularities in the wall of the borehole. The bow springs may fully collapse to lie generally flat along a portion of the pipe string to facilitate installation of the centralizer into the borehole through the bore of a previously installed or cemented pipe string, for example.
An expandable pipe string may be formed and positioned within an interval of the drilled borehole, and then expanded within the borehole to a larger diameter. A pipe expansion device, such as, but not limited to, a mandrel or rotary expander tool, may be inserted into the bore of the pipe string and forced through the bore to expand the pipe string. Expansion of pipe strings can enable increased capacity of the pipe string to produce reserves, or to accommodate downhole tools. Generally centering an expanded pipe string can provide a uniform annulus around the expanded pipe string in order to obtain a favorable cement liner during the cementing step, but conventional expandable centralizers are likely to be damaged or impaired due to deformation of the conventional centralizer end collars. Expansion of conventional end collars generally causes substantial plastic deformation of the collar material which may result in instability at the collar and bow spring connection. Any restoring force of the bow spring when released to its deployed configuration may impart a substantial twisting force to the compromised collar and bow spring connection that may force the bow spring to twist out of alignment with the collars and the pipe string.
FIG. 1 is a perspective view of a conventional expandable bow spring centralizer 100 received on a pipe string 80 having a diameter D1. The conventional bow spring centralizer 100 has a plurality of angularly distributed bow springs 120, each secured in a generally aligned relationship with the pipe string 80 by the collar and bow spring connections 116 on the pair of conventional end collars 110A, 110B. The conventional centralizer shown in FIG. 1 may be compromised upon expansion, for example due to plastic deformation at the collar and bow spring connections 116.
FIG. 2 is the perspective view of the expanded conventional bow spring centralizer 100′ of FIG. 1 after expansion of the pipe string 80 and the centralizer to an expanded diameter D2. FIG. 2 depicts one embodiment of the substantial plastic deformation that may occur throughout the expanded end collars 110A, 110B, including at the critical collar and bow spring connections 116 on each of the two conventional end collars 110A′, 110B′ of the expanded centralizer 100′. While the plastically deformed collar and bow spring connections 116′ on the centralizer of FIGS. 1 and 2 are shown to be welded, it should be understood that plastic deformation at the collar and bow spring connections may be equally or more compromising where a collar and bow spring connection comprises a fastener and/or a slot and tongue connection because of the stress concentration effects of a hole for receiving a fastener and a slot for receiving a tongue. Similarly, a heat affected zone (HAZ) of a welded connection may contribute to instability resulting from expansion of the conventional collar and bow spring connection.
The need to minimize the thickness of the end collar imposes a significant limitation on the design of the centralizer. The thickness of the centralizer collars prior to expansion within the borehole limits the diameter of the expandable pipe string that can be installed in the borehole through a restriction. Close-tolerance centralizers generally have fully collapsible bow springs and thin-profile end collars. Conventional close-tolerance centralizer end collars may be easily impaired by expansion due to the need to make the collar radially thin enough to fit within a narrow annulus between the expandable pipe string on which the centralizer is received and a restriction, such as, for example, a previously installed pipe string through which the centralizer and the expandable pipe string must pass for installation in a borehole. However, a thin conventional end collar is more susceptible to failure due to expansion, especially at the collar and bow spring connections, due to plastic deformation.
Another challenge associated with expandable bow spring centralizers is related to the need to restrain the bow springs in their collapsed configuration to facilitate installation of the expandable pipe string into the borehole through a restriction, e.g., the bore of a previously installed pipe string, and to the need to deploy the expandable bow spring centralizer within the targeted interval of the borehole to center the expanded pipe string. One method that has been proposed involves the steps of restraining the bow springs of a centralizer in their collapsed configuration using a restraining band of a selected material, and then by corrosively compromising the restraining band with an acid introduced into the borehole after the expandable centralizer and the pipe string on which it is installed are placed in the borehole. Another method that has been proposed involves using downhole actuators, such as hydraulic pistons activated from the surface, to deploy the bow springs when the centralizer and the pipe string are positioned in the targeted interval of the borehole. Still another method, taught in U.S. application Ser. No. 11/828,943 filed on Jul. 26, 2007, involves the use of a prefabricated sacrificial restraining band to restrain the bow springs in their collapsed configuration. The prefabricated band ruptures upon expansion of both the expandable pipe string and the centralizer within the targeted interval of the borehole.
These methods require expensive or complex systems that introduce other problems. For example, an acid that can dissolve the restraining band that restrains the bow springs in their collapsed configuration can be expensive to make and difficult to transport to the well, difficult to place in the borehole at the targeted interval, and it may damage or impair other articles or materials in the borehole that will be exposed to the acid. Similarly, a hydraulic actuator used to deploy the bow springs may add considerable cost and weight to a centralizer, it may consume already limited radial space and thereby limit the size of the expandable pipe string being installed, and additional systems are required to power the hydraulics. Also, in an external restraining band embodiment, the outer diameter of the end collars that connect to and stabilize the bow springs imposes a minimum diameter of a prefabricated restraining band that can be received over the end collars and then positioned on the bow springs to hold them in their collapsed configuration. This limitation on the diameter of a prefabricated restraining band imposed by the outer diameter of the end collars may prevent the bow springs from being restrained in a fully collapsed configuration. As a result, the outer diameter of the collapsed bow springs is greater than it should be, thereby increasing the running and starting forces encountered during installation of the pipe string in the borehole.
What is needed is an expandable bow spring centralizer with close-tolerance expandable collars that expand with substantially reduced plastic deformation of the material at the collar and bow spring connections. What is needed is an expandable close-tolerance bow spring centralizer collar that maintains stable collar/bow spring connections after expansion of a pipe string on which the centralizer is received. What is needed is an expandable centralizer comprising close-tolerance expandable collars that are adapted to expand in diameter primarily through sliding rather than primarily through plastic deformation. What is needed is an expandable bow spring centralizer with bow springs that can be collapsed and restrained in the fully collapsed configuration by one or more restraining bands, and deployed to an expanded configuration by sacrificial failure of the band upon expansion of a pipe string upon which the centralizer is received. What is needed is an expandable centralizer that is adapted to be collapsed and restrained in its collapsed configuration using structures that introduce no unwanted materials into the borehole. What is needed is a restraining band to restrain the bow springs of a centralizer in their collapsed configurations that can be of a diameter that is smaller than the end collar of the centralizer.